This part is incorrect:

        <fx:fileset dir="dist" includes="*.jar"/>
        <fx:fileset dir="dist/lib" includes="*.jar"/>

Should be either
        <fx:fileset dir="dist" includes="*.jar"/>
        <fx:fileset dir="dist" includes="lib/*.jar"/>
or
        <fx:fileset dir="dist" includes="**/*.jar"/>

In case of this specific example first notation (2 lines) is preferable as we also show what to do with "other" jar types.